In many wireless communication stations, such as cellular/PCS base stations, RF communication signals are received by an antenna at the top of a tower, routed down to equipment at the base of a tower (“base station”), downconverted from RF, and demodulated to audio. For further processing, the signals are then routed back to a Mobile Switching Center (MSC), Central Office (CO), or other facility, using another wireless communication link or using a wired link, such as a T1 line. This routing back to the MSC/CO is referred to as backhaul.
About 80% of all base stations route the signals back to the MSC/CO via a microwave backhaul link. That is, the base station signals are converted to microwave backhaul. More specifically, at the base station, the audio signals are arranged or stacked and are upconverted to IF. The signals are then remodulated, usually using a different modulation scheme, and converted to a microwave frequency. They are then amplified and transmitted out via a microwave antenna or dish. The modulation and other signal processing is traditionally handled at the ground level of the tower, while the conversion to a microwave spectrum may be handled on the ground or on the tower. The primary reason for this whole complicated loop is that the RF Cellular/PCS signals are spread apart in the RF band, due to frequency re-use, and often occupy distinct bands, like a comb. For example, in a typical TDMA system, total comb bandwidth is around 12.5 MHz. However, the microwave link bandwidths are often much narrower.
In order to limit the microwave bandwidth which must be purchased in order to facilitate the backhaul, the base stations have had to utilize expensive modulation/demodulation equipment utilizing digital signal processing or DSP and other supporting circuitry at the base station. For example, the RF wireless communication signals have to be up/down converted and modulated/demodulated down to audio, and then again up/down converted and modulated/demodulated for the microwave backhaul and have to be multiplexed from the RF side with the microwave hardware. For microwave transmissions, the multiplexed audio streams are modulated/demodulated with a different modulation scheme, such as QAM 256, and are up/down converted with respect to the microwave band. The modulation hardware and associated DSP functions are expensive and must be duplicated at all base stations using microwave backhaul. Because the base station hardware takes a larger RF bandwidth and backhauls it over a smaller microwave bandwidth, the base station hardware is considered to provide a spectrum compression mechanism. Without such spectrum compression, it would be necessary to purchase a greater amount of expensive microwave backhaul bandwidth for the backhaul function.
It is therefore desirable to reduce, and even eliminate, the expensive modulation/ demodulation hardware associated with the base station and its backhaul functions. More specifically, it is desirable to eliminate the need for complicated DSP functions and associated hardware at the base station.
It is further desirable to simplify the base station and reduce its overall construction and maintenance costs, while still maintaining the convenient and desirable microwave backhaul function.
It is further desirable to achieve these goals without having to purchase an increased amount of an expensive backhaul bandwidth from the traditional microwave backhaul spectrum.
These goals and improvements, and other features, are addressed by the present invention.